Add slash command example to quickstart (#639)

Author: hypergonial

docs/quickstart.rst

@@ -5,13 +5,13 @@
 .. currentmodule:: discord
 
 Quickstart
-============
+==========
 
 This page gives a brief introduction to the library. It assumes you have the library installed,
 if you don't check the :ref:`installing` portion.
 
 A Minimal Bot
----------------
+-------------
 
 Let's make a bot that responds to a specific message and walk you through it.
 
@@ -40,7 +40,7 @@ It looks something like this:
 Let's name this file ``example_bot.py``. Make sure not to name it ``discord.py`` as that'll conflict
 with the library.
 
-There's a lot going on here, so let's walk you through it step by step.
+There's a lot going on here, so let's walk you through it step by step:
 
 1. The first line just imports the library, if this raises a `ModuleNotFoundError` or `ImportError`
    then head on over to :ref:`installing` section to properly install.
@@ -77,3 +77,41 @@ On other systems:
     $ python3 example_bot.py
 
 Now you can try playing around with your basic bot.
+
+A Minimal Bot with Slash Commands
+---------------------------------
+
+As a continuation, let's create a bot that registers a simple slash command!
+
+It looks something like this:
+
+.. code-block:: python3
+
+    import discord
+
+    bot = discord.Bot()
+
+    @bot.event
+    async def on_ready():
+        print(f"We have logged in as {bot.user}")
+
+    @bot.slash_command(guild_ids=[your, guild_ids, here])
+    async def hello(ctx):
+        await ctx.respond("Hello!")
+
+    bot.run("your token here")
+
+Let's look at the differences compared to the previous example, step-by-step:
+
+1. The first line remains unchanged.
+2. Next, we create an instance of :class:`.Bot`. This is different from :class:`.Client`, as it supports
+   slash command creation and other features, while inheriting all the features of :class:`.Client`.
+3. We then use the :meth:`.Bot.slash_command` decorator to register a new slash command. 
+   The ``guild_ids`` attribute contains a list of guilds where this command will be active. 
+   If you omit it, the command will be globally available, and may take up to an hour to register.
+4. Afterwards, we trigger a response to the slash command in the form of a text reply. Please note that
+   all slash commands must have some form of response, otherwise they will fail.
+6. Finally, we, once again, run the bot with our login token.
+
+
+Congratulations! Now you have created your first slash command!